In medical ultrasonic imaging apparatuses, ultrasonic wave is irradiated into an object to be examined and two-dimensional distribution of blood flow is displayed in a color image by power of a Doppler signal. Being different from a method of displaying blood flow by frequency of a Doppler signal, a method of displaying blood flow by power of a Doppler signal cannot display direction and speed of blood flow or speed dispersion but has unique function that existence of blood flow and its strength can be displayed at high sensitivity and high S/N ratio. In view of the unique function, this method becomes one of useful blood flow display methods.
In such an apparatus, as described in reference "Diagnostic Imaging", December 1993, pp 66-69, a blood flow display region is provided in a part of a B-mode image display region and a blood flow image is displayed in the blood flow display region. The blood flow display region can be moved freely in the B-mode image display region using an operation device, thereby a blood flow image in a desired region under study can be displayed.
The blood flow image is displayed by color corresponding to power of a Doppler signal, for example, by violet, red, orange, yellow in the rising order of the power. A portion without blood flow is displayed by color other than the above, e.g., by blue, and can be clearly distinguished from a portion with blood flow. Since a blood flow image by power of a Doppler signal has a good S/N ratio, a portion without blood flow is colored entirely with uniform blue.
Therefore as shown in FIG. 6 for example, when a tumor image 83 is displayed in a B-mode image display region 82 on a display surface 81 of an image display, in the case that a blood flow display region 84 is moved to a portion including the tumor image 83 so as to display blood flow of the portion, if the blood flow display region 84 overlaps with the tumor image 83 on the way of the moving as shown in FIG. 7, the tumor image 83 is hidden and comes out of sight and the target cannot be confirmed.
In general, since an ultrasonic transducer, e.g., an ultrasonic probe is made touch an object to be examined by an operator, its direction is liable to vary. Therefore the deviation of the direction of the transducer must be corrected while viewing the screen, so that the tumor image 83 always appears on the display surface. However, if the target comes out of sight at the way of the moving as above described, even when the direction of the transducer is deviated, it cannot be corrected. Consequently it becomes unclear that the blood flow displayed on the blood flow display region 84 be blood flow at the desired portion certainly.